Solid state devices are typically made from semiconductor material. Semiconductor material is a material that has a resistance that lies between that of a conductor and an insulator. In creating a device in semiconductor material, device regions are formed to be either N conductivity type (N type) or P conductivity type (P type). The N type semiconductor material is doped with a donor type impurity that generally conducts current via electrons. P type semiconductor material is doped with acceptor-type impurities that conducts current via hole migration. One example of a solid state device is a bipolar NPN transistor. A bipolar NPN transistor is made of a N type emitter, a P type base and a N type collector.
One common method of creating a P type base is by introducing Boron dopants into a select region of the semiconductor material. More recently, the use of Indium has been used to create the P type base. The use of Indium provides an improved beta early voltage product (hFE−VA product). Wherein hFE is the transistor gain (beta) and VA is early voltage. Basically, the early voltage is a measure of how rapidly the depletion layer from a base-collector junction spreads into a base thereby changing the net base doping in non-depleted portions of the base. A high beta early voltage product is achieved by minimizing the spread of the depletion layer into the base. Indium dopants are effective in holding back the depletion layer to achieve an improved beta early voltage product.
In particular, the improved beta early voltage product acquired with the use of Indium results in increased collector currents and collector-emitter gains as compared to boron implanted transistor bases. The increase in beta early voltage product arises from the fact that Indium resides farther from the band edge than Boron. Because of this, Indium in neutral regions partially freezes-out. That is, a lot of the Indium dopant (which in this case is an acceptor) does not accept an electron from the valence band thereby forming holes. In the depleted regions (relevant to its performance here) the Indium in the base-collector depletion region is totally ionized. The result is, that beta (which is set by the low fraction of ionized indium in the non-depleted portions of the base) is high while the spread of the base-collector depletion layer into the base (which is set by the higher fraction of ionized indium in the depleted layer) is low compared to base regions formed with Boron implant. The hole concentration of Indium is given by the following hole concentration equation:
  p  =            2      ⁢              N        A                    1      +                        1          +                      4            ⁢                          g              A                        ⁢                                          N                A                                                              N                  V                                ⁢                                  exp                  ⁡                                      (                                                                                            E                          v                                                -                                                  E                          A                                                                    kT                                        )                                                                                          
In the hole concentration equation, p is the hole concentration, NA is the indium concentration, gA is a degeneracy factor which is approximately equal to 4, EV is a valence band edge energy, EA is an acceptor (Indium) ionization energy, k is Boltzman's constant and T is the absolute temperature.
One limitation of a device made with an Indium base is that the device will have a high base resistance. The base resistance is inversely proportional to the integrated doping in the neutral or un-depleted base region. A high base resistance leads to high noise in the device. So even though you can achieve a relatively high beta early voltage product in a device with a base doped with Indium, the relatively high base resistance created by the Indium dopant limits the applicability of the device. It is desired in the art to have a device with a base that has relatively high beta early voltage product and relatively low base resistance.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a device with a base that has relatively high beta early voltage product and relatively low base resistance.